I haven't got one here to confirm, but I would assume it'll be the same way as any other PIC with normal on-board EEPROM...

WRITE Address,Mybyte

where 'Address' is any value between 0 and 127 for this PIC, and MyByte is a normal byte variable.

It's an easy mistake to get the Address and MyByte transposed just make sure you're not doing... WRITE MyByte,Address

Melanie